As for your out-of-pocket costs, it really depends on the property and your specific situation, but budgeting around 5-10% of the purchase price for these extras is a good starting point.

You can buy a new one brand name for around 300-700 dollars, depending on brand, and get it installed for $200, or do it yourself with some basic skills and youtube video.

Depending on the area, you can get 1k or more per room.

The kinds of attributes you seek will depend on your approach.Given your youth, take into account the potential for house hacking.

Depending on the loan product used - to purchase a non owner occupied investment property you will need a 15-20% down payment plus closing costs.

Refinance raises the payments potentially & the higher interest rate may improve your tax write offs depending on your bracket.  

Depending on the size of your complex you'll likely need the HOA board approval to do this. 

As a former leasing manager, I would not have charged the reletting fee, and probably returned your deposit, but all that depends on the wording of the lease and property condition.

Buy a neutralizer, spread it around heavy, poly film it, come back in 24 hours, ozonate if there is still smell (likely for a couple days or more depending on smell and ozone capacity of machine), then you’re probably sanding and throwing down whatever stain /polyurethane you desire.

It really depends on the area and type of property, as in many neighborhoods people would prefer a bigger yard over a pool that they have to pay maintenance for.